1. Field of the Invention
This invention relates to the manufacture of bottles or containers of thermoplastic materials for the retention of fluids under pressure, such as carbonated beverages or the like.
2. Description of the Prior Art
Recently, various thermoplastic materials have been developed which are capable of preventing the migration of carbon dioxide (CO.sub.2) therethrough and are blow-moldable into suitable container configurations. Such materials include polyethyleneterephthalate or PET; or nitrile based resins known as LOPAC, a registered trademark of Monsanto Company, or nitrile-group-containing monomers of the type disclosed in U.S. Pat. No. 3,873,660.
Such a bottle or container generally consists of a shoulder portion with a cap-receiving finish, a side wall or main body portion, and a bottom wall joined to the side wall. Pressure retaining bottles are generally of cylindrical overall contour, but the present invention is applicable to bottles of other than cylindrical contours. For purposes of simplicity of description, such terms as "cylindrical", "annular", etc. are herein utilized, but it should be understood that these terms are merely descriptive, not limiting in a geometric sense.
One primary problem which is encountered in blow-molding thermoplastic materials to form bottles or containers capable of retaining CO.sub.2 and other gases under pressure resides in the provision of a bottom shape capable as serving as a bottle support while resisting deformation under pressure to thereby result in a container which is dimensionally stable. One suitable bottom shape is a simple, outwardly hemispherical shape. However, a container employing a hemispherically shaped bottom obviously requires a separately applied, outer peripheral support to enable the bottle to stand upright. A less expensive, and more practical shape results from the inversion of the outwardly hemispherical shape to an outwardly concave or "champaign bottom" shape. The transition region located at the juncture of the cylindrical bottle side wall with the inverted, concave bottom forms a seating ring upon which the bottle is supported in an upright position. Much effort has been devoted to the design of inverted, concave bottoms of this type, and many different methods and many different molds have been developed.
To reduce the creep characteristic of polymeric materials under internal pressure, the material is orientated during the bottle formation, requiring blowing at a reduced temperature. Attempts to form a concave bottom by directly inflating a parison in a blow mold of the final bottle shape have failed. Under these blowing conditions, the material simply "bridges over" the sharp curvatures required in the mold to form an adequate seating ring, and the material stretches and thins out in the region where the greatest strength is required. As a result, seating rings deform under internal pressure to reduce the seating ring diameter and to change the pressure-resistant characteristic of the concave bottom.
It has been proposed that an initially outwardly convex bottom be blown which is then inverted to form a final concave bottom. Those methods and apparatus heretofore proposed either (1) require the utilization of a separate inversion mold and reheating of the initial bottom, or (2) simply push a convex die against the outwardly convex bottom. Neither technique has solved the problems inherent in the requirements of sharp curvatures in the transition zone and of adequate material thickness at the seating ring.
One solution to the problem is disclosed in U.S. Pat. No. 4,134,510. A blowable pre-form is initially expanded against a composite mold surface defined by the end faces of a plurality of concentric tubes surrounding a central actuating rod. The rod and the tubes are initially telescopically positioned to define the composite concave surface, so that a first convex bottom is blown. Subsequently, the rod and tubes are actuated telescopically to progressively invert the convex bottom to a concave shape. The end faces of the tubes may be grooved to define reinforcing ribs in the concave bottom wall, if desired. Such a container has a concave bottom wall of improved resistance to deformation under internal pressure. This is accomplished by forming a support ring at the junction of a pair of oppositely directed inner and outer bottom walls, the juncture of the wall defining an included angle which is equal to or less than 90.degree. and the internal radius of the support ring which is equal to or less than four times the thickness of the walls.
One problem with push-up type freestanding containers under internal pressure is that the inside wall joining the seating ring has a tendency to roll out and the radius of the seating ring tends to shrink such that the bottom tends to grow longer. In the extreme case, the deformation due to the internal pressure leads to rocker bottom. The deformation is mainly caused by a low bending moment at the seating ring area, and, as a result, requires a thicker wall in the seating ring area to resist such deformation. The inability to distribute more material in the seating ring region in the formation of an oriented container is the main reason that a large functional seating ring is difficult to fabricate.